There's a bit of controversy regarding the unlocking of this map as it appears a glitch has prevented some people from unlocking the map as it was originally intended. The way it has been done successfully is as follows:

Complete single-player on any difficulty.

Create a new profile.

Begin a co-op game on the last level with the newly-created profile as the second player.

When you reach a section where you fly a Banshee, the new profile must perform a barrel roll and a loop-the-loop.

Finish the campaign and the new profile can access the Foundation map when hosting split-screen, System Link, or Xbox Live games.

Foundation is a small, enclosed map that defaults with no vehicles but plenty of deadly weapons. An Energy Sword lies in the center of the map with a rocket launcher waiting on one of the upper balconies. A pair of turrets look down on the courtyard as well to provide some extra firepower.
Gaining access to the upper level is easy enough. There are several stairs as well as multiple air elevators. For some good chuckles, put a Warthog or Ghost one the map and drive it onto the air vent for a quick ride to the top. The upper tier features multiple dead-end rooms that serve as excellent places for up-close gun battles.


-Originally from IGN
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Halo 2 Egg: Grunt B-day party
Game: Halo 2

How it Works:

1. Complete the lever "The Arbiter" in any level on any difficulty. Then go back to the same level of Legendary.

2. Play until you reach the part where you must follow the "Heretic leader" in a banshee.

3. Get into the banshee and fly straight down where you will find 3 large pillars that support the structure.

4. On top of the middle one you will find few grunts dancing around an oddball.

5. Pickup the oddball using "X".

6. Now whenever you shoot anyone in the head, they will explode with the force of a plasma grennade.

7. The party is extremely tough to find so it may take you a while, but is worth all the trouble.

-Originally from IDNews
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And: https://halosm.bungie.org/tipsntricks/eggs.html (HALO 1)